TM 5-820-4/AFM 88-5, Chap 4
CHAPTER 5
EROSION CONTROL AND RIPRAP PROTECTION
the duration and Froude number of the design
5-1. General.
flow at the culvert outlet. These equations and
those for the maximum depth, width, length and
channels will be provided with riprap protection
volume of scour and comparisons of predicted and
to prevent erosion. Two general types of channel
observed values are discussed in chapter 10, TM
instability can develop downstream from a culvert
5-820-3/AFM 88-5, Chapter 3. Examples of rec-
and stormdrain outlet. The conditions are known
ommended application to estimate the extent of
as either gully scour or a localized erosion referred
scour in a cohesionless soil and several alternate
to as a scour hole. Distinction between the two
schemes of protection required to prevent local
conditions of scour and prediction of the type to
scour downstream of a circular and rectangular
be anticipated for a given field situation can be
outlet are illustrated in Practical Guidance for De-
made by a comparison of the original or existing
sign of Lined Channel Expansions at Culvert Out-
slope of the channel or drainage basin down-
lets, Technical Report H-74-9.
stream of the outlet relative to that required for
stability.
5-2. Riprap protection,
b. Gully scour is to be expected when the Froude
a. Riprap protection should be provided adja-
number of flow in the channel exceeds that re-
cent to all hydraulic structures placed in erosive
quired for stability. It begins at a point down-
materials to prevent scour at the ends of the struc-
stream where the channel is stable and pro-
ture, The protection is required on the bed and
gresses upstream. If sufficient differential in
banks for a sufficient distance to establish velocity
elevation exists between the outlet and the sec-
gradients and turbulence levels at the end of the
tion of stable channel, the outlet structure will be
riprap approximating conditions in the natural
completely undermined. Erosion of this type may
channel. Riprap can also be used for lining the
be of considerable extent depending upon the lo-
channel banks to prevent lateral erosion and un-
cation of the stable channel section relative to
desirable meandering. Consideration should be
that of the outlet in both the vertical and down-
given to providing an expansion in either or both
stream directions.
the horizontal and vertical direction immediately
c. A scour hole or localized erosion is to be ex-
downstream from hydraulic structures such as drop
pected downstream of an outlet even if the down-
structures, energy dissipators, culvert outlets or
stream channel is stable. The severity of damage
other devices in which flow can expand and dis-
to be anticipated depends upon the conditions ex-
sipate its excess energy in turbulence rather than
isting or created at the outlet. In some instances,
in a direct attack on the channel bottom and sides.
the extent of the scour hole may be insufficient
to produce either instability of the embankment
b. There are three ways in which riprap has
or structural damage to the outlet. However, in
been known to fail: movement of the individual
many situations flow conditions produce scour of
stones by a combination of velocity and turbu-
the extent that embankment erosion as well as
lence; movement of the natural bed material
structural damage of the apron, end wall, and cul-
through the riprap resulting in slumping of the
vert are evident.
blanket; and undercutting and raveling of the rip-
d. The results of research conducted at US Army
rap by scour at the end of the blanket. Therefore,
Engineer Waterways Experiment Station to de-
in design, consideration must be given to selection
termine the extent of localized scour that may be
of an adequate size stone, use of an adequately
anticipated downstream of culvert and storm-drain
graded riprap or provision of a filter blanket, and
outlets has also been published. Empirical equa-
proper treatment of the end of the riprap blanket.
tions were developed for estimating the extent of
the anticipated scour hole based on knowledge of
53. Selection of stone size. There are curves
the design discharge, the culvert diameter, and
available for the selection of stone size required
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